Tonometer

Abstract

A tonometer having a construction whereby thermal expansion or contraction of the material forming the tonometer casing will have substantially no influence upon the precision of measurement of the instrument. The tonometer comprises a casing, a bellows mounted in a bottom portion of the casing, a rod resting on the bellows adapted to be axially shifted as the bellows expands and contracts, a base plate mounted above the bellows in the main portion of the casing, and an amplifying mechanism including a pointer mounted on the base plate in operative association with the rod for swinging the pointer. The base plate includes at least one leg forming an extension thereof which is fixed with respect to the casing so that the leg or legs constitute the sole means by which the base plate is fixedly mounted with respect to the casing. The rod rests on the bellows at a position closely adjacent to a position at which the base plate is fixed with respect to the casing.

What is claimed is: 
     
       1. In a tonometer including a casing defining an interior, said casing having main and bottom portions, bellows means mounted in the interior of said casing bottom portion, rod means resting on said bellows means adapted to be axially shifted by said bellows means as the latter expands and contracts, a base plate having a thickness mounted in the interior of said casing main portion, a pointer rotatably mounted on said casing, and amplifying means mounted on said base plate in operative assocation with said rod means for swinging said pointer an amount corresponding to a degree of expansion or contraction of said bellow means, the improvement comprising: said base plate includes at least one leg means forming an extension thereof, said at least one leg means of said base plate being fixed with respect to said casing with said leg means constituting the sole means by which said base plate is fixedly mounted with respect to said casing, and   wherein said rod means rests on said bellows means at a position closely adjacent to a position at which said base plate is fixed with respect to said casing.   
     
     
       2. The combination of claim 1 wherein said at least one leg means of said base plate has at least one opening formed therein and said casing has a corresponding number of openings formed therein at positions corresponding and adapted to align with respective openings formed in said base plate leg means, and wherein fastening means extend through pairs of aligned base plate and casing openings for fixedly mounting said base plate with respect to said casing. 
     
     
       3. The combination of claim 2 wherein said fastening means comprise threaded members adapted to be threaded in respective pairs of aligned base plate and casing openings. 
     
     
       4. The combination of claim 1 further including means for restraining a portion of said base plate opposed from said base plate leg means against lifting away from said casing beyond a predetermined amount while allowing said base plate portion to remain non-fixed to said casing. 
     
     
       5. The combination of claim 4 wherein said restraining means comprise: a notch formed at an upper portion of said base plate having a radius of curvature, a seat provided on said casing at a position corresponding to the position of said notch, said seat having a threaded bore formed therein, a threaded member having a head and a threaded shank having a step portion formed therein below said head, said step portion having a radius of curvature which is less than the radius of curvature of said notch and an axial length which is slightly larger than the thickness of said base plate in the region of said notch, and wherein said threaded member is threaded within said threaded bore, whereby the head of the threaded member restrains said upper portion of said base plate against lifting away from the casing beyond a predetermined amount, while allowing said base plate portion to remain non-fixed to said casing. 
     
     
       6. The combination of claim 1 wherein said bottom portion of said casing comprises a bottom connector portion integral with said casing main portion and a separable bottom main portion in which said bellow means are mounted adapted to be connected to said bottom connector portion, and wherein said bottom connector portion extends from said casing main portion and terminates at a region proximate to said position at which said base plate is fixed with respect to said casing, and   wherein said separable bottom main portion is made of a material having a coefficient of thermal expansion which is approximately equal to that of the bellows means mounted therein.   
     
     
       7. The combination of claim 1 wherein at least said rod means is made of material having a coefficient of thermal expansion higher than that of the base plate so that an influence deriving from thermal expansion of said casing is offset by a difference in thermal expansion between said rod means and said base plate.